The present invention generally relates to the field of GPS systems and particularly to acquisition of PN sequences in GPS systems.
In GPS P(Y) code acquisition, the PN spread spectrum sequence is a one week long sequence having a rate of 10.23 MHz (6,187,104,000,000 bits). With no knowledge of an external time reference and with present digital correlator technology, it is impractical to accomplish a direct acquisition of a very large PN sequence such as the GPS P(Y) code in a reasonable amount of time (e.g., less than ten minutes).
Typical P(Y) code acquisition techniques involve correlating a reference PN signal with the transmitted PN sequence at all possible code positions. This correlation generally involves sequentially advancing the reference PN sequence with respect to the transmitted PN sequence until correlation is detected (indicating identity of the sequences). However, with very large length PN sequences and no knowledge of an external time reference, it is often impractical to sequentially correlate at all code positions in a reasonable amount of time due to the finite number of correlation processors available. Thus, there lies a need to provide a system and method for acquiring a relatively large, finite PN sequence in a reasonable period of time without knowledge or with poor knowledge of an external time reference.
The present invention is directed to a method for acquiring a pseudorandom (PN) sequence. In one embodiment, the method includes steps for sectioning a reference PN sequence into a predetermined number of subsequences, (with possible gaps between the subsequences) receiving the PN sequence from a remote source, correlating the received PN sequence with each of the predetermined number of subsequences simultaneously, and in the event the correlating step produces correlation between the received PN sequence and one of the predetermined number of subsequences, acquiring the received PN sequence.
The present invention is further directed to a GPS receiver for acquiring a GPS P(Y) code signal without knowledge or with poor knowledge of an external time reference. In one embodiment, the receiver includes a first filter and amplifier for selecting a predetermined frequency band of a received GPS signal and for amplifying the filtered signal, a first converter for converting the received GPS signal into an intermediate frequency signal, a second filter and amplifier for filtering and amplifying the intermediate frequency signal, a second converter for converting the filtered and amplified intermediate frequency signal into a digital signal representative of the GPS signal, and a processor for receiving the digital signal, the processor being configured to simultaneously correlate the digital signal with a plurality of subsequences of a reference signal such that the GPS signal is acquired upon correlation of the digital signal with one of the plurality of subsequences of the reference signal.
It is an object of the present invention to provide a method and appartus for acquiring large PN sequences in a short period of time.
It is a feature of the present invention to acquire a large PN sequence in six minutes or less.
It is a feature of the present invention to acquire a large PN sequence by holding the reference PN sequence fixed and waiting until the received PN sequence generates a match between the sequences.
It is an advantage of the present invention to acquire a large PN sequence without a time reference.
It is an advantage of the present invention to acquire a large PN sequence with a poor time reference.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed.
The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the general description, serve to explain the principles of the invention.